1. Field of the Invention
The invention relates to a method for molding a silicone elastomer into a fiberglass honeycomb panel in order to fabricate a structure to provide thermal protection for space re-entry vehicles.
2. Description of the Prior Art
Prior art methods of fabricating heat insulation structures of the type described require the filling of each honeycomb cell individually after it has been bonded to a structural support plate. This method generally results in a varying cell to cell density of the material as well as variations in total fill depth. The individual cell filling method is both time consuming and costly and generally requires several filling and X-ray inspection operations in order to obtain a void free structure. When mechanical tamping is utilized to force the ablation composition into the honeycomb cells, the procedure is further restricted since the tamping technique is not applicable to low density or high viscosity ablation formulation. In addition, stresses are developed in the honeycomb cell walls during the filling operation.
Several prior art filling techniques have been developed to simultaneously fill a complete honeycomb structure. U.S. Pat. No. 3,585,100 to Greenlees and U.S. Pat. No. 3,425,885 to Newcomb et al are examples of this simultaneous fill process. Greenlees for example, discloses a method for fabricating a panel structure comprising resin and crushed quartz by pressing the honeycomb structure into the filled mold mixture and then placing the mold in a vacuum chamber and applying heat in order to attain a low viscosity state. Under this method uneven filling and voids may result from air trapped in the honeycomb structure in the mold filling process as well as in the pressure step which forces the honeycomb into the mold mixture. Newcomb teaches a vacuum filling technique of a de-aerated ablative filling material in which the honeycomb structure is held stationary and the ablative material is forced into the honeycomb structure. Ablative materials are, however, pressure load rate sensitive, and forcing such materials at high rates or under large loads results in the imposition of lateral forces on the cell walls of the honeycomb structure, thus tending to shear the bonded honeycomb nodes. These side forces are particularly prevalent in ablation materials in the density range of 10-40 lbs/ft.sup.3. In addition, the bonding of the honeycomb panel to a rigid support panel or "face sheet" tends to introduce additional stresses in the cell walls when filling with pressure load rate sensitive materials. Possible honeycomb damage is a major disadvantage in prior art filling techniques for ablative materials, particularly those having a density range between 10-40 lbs/ft.sup.3.